U.S. patent application number 13/903277 was filed with the patent office on 2014-02-27 for optical sheet and display device.
This patent application is currently assigned to Dai Nippon Printing Co., Ltd.. The applicant listed for this patent is Dai Nippon Printing Co., Ltd.. Invention is credited to Fumihiro ARAKAWA, Kohei KOMIZO, Takashi KURODA.
Application Number | 20140055854 13/903277 |
Document ID | / |
Family ID | 49846191 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140055854 |
Kind Code |
A1 |
ARAKAWA; Fumihiro ; et
al. |
February 27, 2014 |
OPTICAL SHEET AND DISPLAY DEVICE
Abstract
An optical sheet (40) is used in a display device for switchably
displaying a two-dimensional image and a naked-eye visible
three-dimensional image. The optical sheet (40) includes: a first
layer (51) including a thermoplastic resin; and a second layer (52)
laminated to the first layer. The first layer is an optically
anisotropic An optical interface (55) for changing the traveling
direction of light of one polarization component is formed between
the first layer (51) and the second layer (52).
Inventors: |
ARAKAWA; Fumihiro;
(Shinjuku-Ku, JP) ; KURODA; Takashi; (Shinjuku-Ku,
JP) ; KOMIZO; Kohei; (Shinjuku-Ku, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dai Nippon Printing Co., Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Dai Nippon Printing Co.,
Ltd.
Tokyo
JP
|
Family ID: |
49846191 |
Appl. No.: |
13/903277 |
Filed: |
May 28, 2013 |
Current U.S.
Class: |
359/462 ;
359/489.11 |
Current CPC
Class: |
G02B 30/25 20200101;
G02B 5/3083 20130101; G02B 30/27 20200101 |
Class at
Publication: |
359/462 ;
359/489.11 |
International
Class: |
G02B 27/26 20060101
G02B027/26; G02B 5/30 20060101 G02B005/30 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2012 |
JP |
2012-122294 |
Claims
1. An optical sheet comprising: a first layer including a
thermoplastic resin, the first layer being optically anisotropic;
and a second layer which is laminated to the first layer and which
forms, between it and the first layer, an optical interface for
changing a traveling direction of light of one polarization
component, wherein the optical sheet controls a traveling direction
of light depending on a polarization state of the light and is used
in a display device for switchably displaying a two-dimensional
image and a naked-eye visible three-dimensional image.
2. The optical sheet according to claim 1, wherein a material of
the first layer has a glass transition temperature of not less than
100.degree. C.
3. The optical sheet according to claim 1, wherein the
thermoplastic resin is a polyethylene naphthalate resin.
4. The optical sheet according to claim 1, wherein an in-plane
birefringent index .DELTA.n of the first layer is not less than
0.13.
5. The optical sheet according to claim 1, wherein light of the
other polarization component, traveling in a normal direction of
the optical sheet before entering the optical sheet, travels in a
direction at an angle of not more than 2 degrees with respect to
the normal direction of the optical sheet after passing through the
optical sheet.
6. The optical sheet according to claim 1, wherein a dimensional
stability of the optical sheet, measured according to JIS C2151
using a heating conditions of 150.degree. C., 30 minutes, is not
more than 2%.
7. The optical sheet according to claim 1, wherein the second layer
is optically isotropic.
8. The optical sheet according to claim 1, wherein an electric
dipole moment of the first layer is higher than an electric dipole
moment of the second layer.
9. A display device comprising: the optical sheet according to
claim 1; and an image display unit disposed opposite to the optical
sheet and configured to be capable of emitting light of one
polarization component for forming a three-dimensional image and
light of the other polarization component for forming a
two-dimensional image, wherein the display device switchably
displays a two-dimensional image and a naked-eye visible
three-dimensional image.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Patent
Application No. 2012-122294, filed on May 29, 2012, the disclosure
of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a display device for
switchably displaying a two-dimensional image and a naked-eye
visible three-dimensional image, and to an optical sheet for use in
the display device and for controlling the traveling direction of
light depending on the polarization state of the light.
[0004] 2. Description of the Related Art
[0005] A display device which displays three-dimensional images
that can be viewed with the naked eye has been developed, as
disclosed e.g. in WO 03/015424 A2. Such a display device is
configured to be capable of switching between a two-dimensional
image formed on a display surface and a three-dimensional image,
having a sense of depth, which can be viewed also at a position at
a distance from the display surface.
[0006] In the display device disclosed in WO 03/015424 A2, a
two-dimensional image is formed by one linear polarization
component, while a three-dimensional image is formed by the other
linear polarization component. The display device has a
birefringent lens which exerts a lens function only on light of the
other linear polarization component that forms a three-dimensional
image. Light of the other linear polarization component, emitted
from a pixel for forming a left-eye image, is focused by the
birefringent lens on the left-eye position of a viewer. Similarly,
light of the other linear polarization component, emitted from a
pixel for forming a right-eye image, is focused by the birefringent
lens on the right-eye position of the viewer. Consequently, the
viewer views the left-eye image with the left eye while viewing the
right-eye image with the right eye. The viewer can thus visually
perceive a three-dimensional image.
[0007] The birefringent lens has an optically anisotropic layer and
an optically isotropic layer, disposed adjacent to each other.
[0008] The refractive indices of the optically anisotropic layer
and the optically isotropic layer are the same in the direction of
vibration of one linear polarization component (polarized direction
of one linear polarization component) and are different in the
direction of vibration of the other linear polarization component
(polarized direction of the other linear polarization component).
Accordingly, only light of the other linear polarization component
changes its traveling direction at the interface between the
optically anisotropic layer and the optically isotropic layer.
[0009] The optically anisotropic layer of the birefringent lens
exhibits optical anisotropy due to the presence of an oriented
liquid crystal material in the layer. Because of the presence of
the liquid crystal material, the birefringent lens lacks stability,
especially thermal stability. This imposes restrictions on the
environment in which the birefringent lens is used and on the
environment in which a display device having the birefringent lens
is installed.
SUMMARY OF THE INVENTION
[0010] The present invention has been made in view of the above
situation in the related art. It is therefore an object of the
present invention to improve stability of an optical sheet for
controlling the traveling direction of light depending on the
polarization state of the light.
[0011] In order to achieve the object, the present invention
provides a first optical sheet, said sheet comprising:
[0012] a first layer including a thermoplastic resin, the first
layer being optically anisotropic; and
[0013] a second layer which is laminated to the first layer and
which forms, between it and the first layer, an optical interface
for changing a traveling direction of light of one polarization
component,
[0014] wherein the optical sheet controls a traveling direction of
light depending on a polarization state of the light and is used in
a display device for switchably displaying a two-dimensional image
and a naked-eye visible three-dimensional image.
[0015] In the first optical sheet according to the present
invention, a material of the first layer may have a glass
transition temperature of not less than 100.degree. C.
[0016] The present invention also provides a second optical sheet,
said sheet comprising:
[0017] an optically anisotropic first layer; and
[0018] a second layer which is laminated to the first layer and
which forms, between it and the first layer, an optical interface
for changing a traveling direction of light of one polarization
component,
[0019] wherein a material of the first layer may have a glass
transition temperature of not less than 100.degree. C., and
[0020] wherein the optical sheet controls a traveling direction of
light depending on a polarization state of the light and is used in
a display device for switchably displaying a two-dimensional image
and a naked-eye visible three-dimensional image.
[0021] The present invention also provides a third optical sheet,
said sheet comprising:
[0022] an optically anisotropic first layer; and
[0023] a second layer which is laminated to the first layer and
which forms, between it and the first layer, an optical interface
for changing a traveling direction of light of one polarization
component,
[0024] wherein the first layer contains no liquid crystal, and
[0025] wherein the optical sheet controls a traveling direction of
light depending on a polarization state of the light and is used in
a display device for switchably displaying a two-dimensional image
and a naked-eye visible three-dimensional image.
[0026] In any one of the first to third optical sheets according to
the present invention, the thermoplastic resin may be a
polyethylene naphthalate resin.
[0027] In any one of the first to third optical sheets according to
the present invention, an in-plane birefringent index .DELTA.n of
the first layer may be not less than 0.13.
[0028] In any one of the first to third optical sheets according to
the present invention, light of the other polarization component,
traveling in a normal direction of the optical sheet before
entering the optical sheet, may travel in a direction at an angle
of not more than 2 degrees with respect to the normal direction of
the optical sheet after passing through the optical sheet.
[0029] In any one of the first to third optical sheets according to
the present invention, a dimensional stability of the optical
sheet, measured according to JIS C2151 using a heating conditions
of 150.degree. C., 30 minutes, may be not more than 2%.
[0030] In any one of the first to third optical sheets according to
the present invention, the second layer may be optically
isotropic.
[0031] In any one of the first to third optical sheets according to
the present invention, an electric dipole moment of the first layer
may be higher than an electric dipole moment of the second
layer.
[0032] In any one of the first to third optical sheets according to
the present invention, the first layer may consist only of the
thermoplastic resin.
[0033] The present invention also provides a display device,
comprising:
[0034] the optical sheet according to claim 1; and
[0035] an image display unit disposed opposite to the optical sheet
and configured to be capable of emitting light of one polarization
component for forming a three-dimensional image and light of the
other polarization component for forming a two-dimensional
image,
[0036] wherein the display device switchably displays a
two-dimensional image and a naked-eye visible three-dimensional
image.
[0037] The present invention also provides a first method for
producing a optical sheet, said method comprising the steps of:
[0038] producing an optically anisotropic first layer by stretching
a resin film made by shaping a thermoplastic resin; and
[0039] producing or laminating a second layer on or to the first
layer,
[0040] wherein the optical sheet controls a traveling direction of
light depending on a polarization state of the light and is used in
a display device for switchably displaying a two-dimensional image
and a naked-eye visible three-dimensional image.
[0041] The present invention also provides a second method for
producing a optical sheet, comprising the steps of: stretching a
first layer and a second layer, the first layer and the second
layer are laminated to each other,
[0042] wherein an electric dipole moment of the first layer is
higher than an electric dipole moment of the second layer.
[0043] The present invention makes it possible to improve stability
of an optical sheet which controls the traveling direction of light
depending on the polarization state of the light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 is a schematic perspective view of a display device,
illustrating one embodiment of the present invention;
[0045] FIG. 2 is a vertical sectional view of the display device of
FIG. 1, illustrating the path of light that forms an image when
displaying a three-dimensional image by means of the display
device;
[0046] FIG. 3 is a vertical sectional view of the display device
of
[0047] FIG. 1, illustrating the path of light that forms an image
when displaying a two-dimensional image by means of the display
device;
[0048] FIG. 4 is a perspective view showing the refractive index
ellipsoids of the first layer and the second layer of an optical
sheet incorporated in the display device of FIG. 1;
[0049] FIG. 5 is a diagram illustrating a method for producing an
optical sheet; and
[0050] FIG. 6 is a diagram illustrating another method for
producing an optical sheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] Preferred embodiments of the present invention will now be
described with reference to the drawings. In the drawings attached
to the present specification, for the sake of illustration and
easier understanding, scales, horizontal to vertical dimensional
ratios, etc. are exaggeratingly modified from those of the real
things.
[0052] FIGS. 1 through 4 are diagrams illustrating an embodiment of
the present invention. Of these, FIG. 1 is a perspective view
showing a display device. FIGS. 2 and 3 are diagrams illustrating
the actions of the display device when it displays a
three-dimensional image and a two-dimensional image, respectively.
FIG. 4 is a perspective view showing the refractive index
ellipsoids of the first layer and the second layer of an optical
sheet.
[0053] The display device 10 of this embodiment can switchably
display a two-dimensional image and a naked-eye visible
three-dimensional image. As shown in FIG. 1, the display device 10
includes an image display unit 15 and an optical sheet 40 which is
disposed so as to face the image display unit 15. The image display
unit 15 is configured to emit light of one linear polarization
component for forming a three-dimensional image and light of the
other linear polarization component for forming a two-dimensional
image. The optical sheet 40 controls the traveling direction of
light depending on the polarization state of the light. More
specifically, the optical sheet 40 controls the traveling direction
of light of the one linear polarization component for forming a
three-dimensional image while maintaining the traveling direction
of light of the other linear polarization component that vibrates
in a direction perpendicular to the direction of vibration of the
one linear polarization component.
[0054] The "two-dimensional image" herein refers to an image which
is viewed two-dimensionally on a display surface 10a, while the
"three-dimensional image" refers to an image having a sense of
depth, which can be viewed also at a position at a distance from
the display surface 10a. The display device 10 of this embodiment
is configured to be capable of displaying a three-dimensional image
by utilizing binocular parallax and motion parallax. As shown in
FIG. 2, when displaying a three-dimensional image, the pixels 21 of
the image forming device 20 of the image display unit 15 are
assigned to those positions where the left eye or the right eye of
a viewer is supposed to be located. Those pixels 21 which are
assigned to the same position form an image to be viewed from the
assigned position. The optical sheet 10, on the other hand,
controls the path of light so that light, emitted from each pixel
21, travels toward the position to which the pixel 21 is assigned
of those positions at which the left eye or the right eye of a
viewer is supposed to be located. Accordingly, the viewer's left
and right eyes view different images, and the viewer perceives a
three-dimensional image. When the viewer changes the viewing
direction, the viewer can view a different three-dimensional image
according to the viewing position.
[0055] The components of the display device 10 will now be
described in greater detail. In the following description, one
linear polarization component that forms a three-dimensional image
will be referred to as "first polarization component" that vibrates
in the x-axis direction (see FIG. 1) parallel to the sheet plane of
the optical sheet 40. The other linear polarization component that
forms a two-dimensional image will be referred to as "second
polarization component" that vibrates in the y-axis direction (see
FIG. 1) perpendicular to the x-axis direction and parallel to the
sheet plane of the optical sheet 40.
[0056] The terms "sheet", "film" and "plate" are not used herein to
strictly distinguish them from one another. Thus, the term "film"
includes a member which can also be called a sheet or plate. An
"optical sheet" is not strictly distinguished from a member called
"optical film" or "optical plate".
[0057] The term "sheet plane (film plane, plate plane, panel
plane)" herein refers to a plane which coincides with the planar
direction of an objective sheet-like (film-like, plate-like,
panel-like) member when taking a wide and global view of the
sheet-like (film-like, plate-like, panel-like) member. In this
embodiment the image forming surface 20a of the image forming
device 20, the panel plane of the liquid crystal display panel 25,
the panel plane of the polarization control device 30, the sheet
plane of the optical sheet 40 and the display surface 10a of the
display device 10 are parallel to each other. The term "front
direction" herein refers to the normal direction of the sheet plane
of the optical sheet 40.
[0058] The terms used herein to specify shapes or geometric
conditions, such as "parallel", "perpendicular", etc., should not
be bound to their strict sense, and should be construed to include
equivalents or resemblances from which the same optical function or
effect can be expected.
[0059] The image display unit 15 includes the image forming device
20 and the polarization control device 30 which transmits light
from the image forming device 20. The polarization control device
30 is disposed between the image forming device 20 and the optical
sheet 40. The image forming device 20 has a large number of pixels
21 arranged in a plane parallel to the image forming surface 20a.
In the illustrated embodiment, the pixels 21 are arranged in a
stripe arrangement. The following description illustrates an
exemplary case where the image forming device 20 forms an image by
using light of the first polarization component. In this case, the
polarization control device 30 maintains the first polarization
state of light, emitted from the image forming device 20, when a
three-dimensional image is to be displayed, or converts the
polarization state of the light into the second polarization state
when a two-dimensional image is to be displayed. However, it is
also possible for the image forming device 20 to emit light of the
second polarization component, and for the polarization control
device 30 to convert the polarization state of light, emitted from
the image forming device 20, into the first polarization state when
a three-dimensional image is to be displayed, or maintains the
second polarization state of the light when a two-dimensional image
is to be displayed.
[0060] In the illustrated embodiment, the image forming device 20
is constructed as a liquid crystal display device. Thus, the image
forming device 20 includes a liquid crystal display panel 25 and a
backlight 24 disposed at the rear of the liquid crystal display
panel 25. The backlight 24 may have any known construction,
including that of the edge-light type or the direct-light type.
[0061] The liquid crystal display panel 25 includes a pair of
polarizing plates 26, 28 and a liquid crystal cell 27 disposed
between the polarizing plates 26, 28. The polarizing plates 26, 28
include polarizers which function to resolve incident light into
two orthogonal polarization components, and transmit one
polarization component and absorbs the other polarization component
perpendicular to the one polarization component. In this embodiment
the lower polarizing plate 26, disposed on the backlight 24 side,
transmits light of the second polarization component, while the
upper polarizing plate 28, disposed on the polarization control
device 30 side, transmits light of the first polarization
component.
[0062] The liquid crystal cell 27 includes a pair of support plates
and liquid crystal molecules (liquid crystal material) disposed
between the support plates. An electric field can be applied to
each pixel area of the liquid crystal cell 27. When an electric
filed is applied to a pixel area, the orientation of the liquid
crystal of the liquid crystal cell 27 changes in the pixel area.
For example, light of the second polarization component, which has
passed through the lower polarizing plate 26, turns its vibration
direction by 90 degrees when it passes through those pixel areas of
the liquid crystal cell 27 to which an electric field is being
applied, whereas light of the second polarization component
maintains its polarization state when it passes through those pixel
areas of the liquid crystal cell 27 to which no electric field is
being applied. Thus, transmission through or absorption and
blocking by the upper polarizing plate 28, disposed on the light
exit side of the lower polarizing plate 26, of light of the second
polarization component, which has passed through the lower
polarizing plate 26, can be controlled by application or no
application of an electric field to each pixel area of the liquid
crystal cell 27. Light of the first polarization component, which
has thus selectively passed through the upper polarizing plate 28
and has been emitted from pixels 21, will form an image.
[0063] The polarization control device 30 will now be described.
The polarization control device 30 basically comprises a pair of a
first electrode 34 and a second electrode 36, and a medium layer 35
disposed between the first electrode 34 and the second electrode
36. The medium layer 35 generates refractive index anisotropy when
a voltage is applied between the pair of the electrodes 34, 36. In
the illustrated embodiment, the first electrode 34, the medium
layer 35 and the second electrode 36 are disposed between a pair of
a first support film 33 and a second support film 37. The first
electrode 34, the medium layer 35 and the second electrode 36 are
supported and protected by the pair of the support films 33, 37.
The following description illustrates a case where the medium layer
is constructed as a liquid crystal layer 35.
[0064] The pair of electrodes 34, 36 and the liquid crystal layer
35 have a size that expands the entire area of the image forming
surface 20a of the image forming device 20. As shown in FIGS. 2 and
3, the liquid crystal layer 35 contains liquid crystal molecules
31. The electrodes 34, 36 are electrically connected to a not-shown
voltage application means. The electrodes 34, 36 are kept at a
predetermined distance from each other e.g. by the use of a spacer
(not shown).
[0065] When the liquid crystal molecules 31 contained in the liquid
crystal layer 35 are typical liquid crystal molecules of the TN
type, the liquid crystal molecules 31 are aligned when a voltage is
applied between the pair of electrodes 34, 36, as shown in FIG. 2.
The first polarization state of light (first polarization
component) from the image forming device 20 is maintained upon
passage of the light through the liquid crystal layer 35 to which a
voltage is being applied. On the other hand, when no voltage is
applied between the pair of electrodes 34, 36, the liquid crystal
molecules 31 are in a 90 degree-twisted or turned state as shown in
FIG. 3. When light from the image forming device 20 passes through
the liquid crystal layer 35 to which no voltage is being applied,
the vibration direction of the light is converted from the x-axis
direction to the y-axis direction, i.e. the light is converted from
the first polarization component to the second polarization
component.
[0066] The above description of the image display unit 15, the
image forming device 20 and the polarization control device 30 is
merely exemplary. Thus, for example, it is also possible to
generate light of the first polarization component by turning the
vibration direction of light from the image forming device 20 by 45
degrees, and to generate light of the second polarization component
by turning the vibration direction of light from the image forming
device 20 by -45 degrees.
[0067] The optical sheet 40 will now be described. As shown in
FIGS. 1 through 3, the optical sheet 40 includes a first layer 51
and a second layer 52 provided adjacent to the first layer 51. In
the illustrated embodiment, the optical sheet 40 further includes a
film layer 43 provided on the second layer 52.
[0068] The film layer 43 may be formed as a single layer or as a
stack of multiple layers. The film layer 43 is expected to exert a
particular function, and forms the outermost light exit-side
surface of the display device 10, i.e. the display surface 10a of
the display device 10. The film layer 43 may comprise at least one
of an antireflective layer (AR layer) having an antireflective
function, an anti-glare layer (AG layer) having an anti-glare
function, an abrasive-resistant hard coat layer (HC layer), an
antistatic layer (AS layer) having an antistatic function, etc.
[0069] The interface between the first layer 51 and the second
layer 52 is formed as a surface having a three-dimensional
(corrugated) pattern. The interface serves as an optical interface
55 which changes the traveling direction of light of at least the
first polarization component. In the illustrated embodiment, the
optical interface 55 between the first layer 51 and the second
layer 52 is constructed as a surface consisting of a plurality of
unit optical interfaces 55a. As shown in FIG. 1, the unit optical
interfaces 55a are arranged in an arrangement direction. Each unit
optical interface 55a extends in a direction not parallel to the
arrangement direction. Particularly in the illustrated embodiment,
the unit optical interfaces 55a are arranged in the x-axis
direction without any space between adjacent unit interfaces, and
each unit optical interface 55a extends linearly in the y-axis
direction. Each unit optical interface 55a has the same shape at
varying positions along the y-axis direction. The unit optical
interfaces 55a all have the same construction.
[0070] As described above, the unit optical interfaces 55a are
designed so that light, emitted from each pixel 21, is directed to
a predetermined position. In the illustrated embodiment, in a
cross-direction parallel to both the front direction and the
arrangement direction of the unit optical interfaces 55a, each unit
optical interface 55a has a convex lens-like contour and focuses a
divergent light flux LF1 (see FIG. 2) from each pixel on a preset
position. The optical interface 55 as an assembly of the unit
optical interfaces 55a forms a lenticular lens.
[0071] The unit optical interfaces 55a and the optical interface
55, shown in the Figures, are merely examples and are capable of
various changes and modifications. For example, the cross-sectional
contour of each unit optical interface 55a may be arbitrarily
changed. Further, the unit optical interfaces 55a may have
different shapes. For example, the optical interface 55 may form a
Fresnel lens. Though the illustrated unit optical interfaces 55a
are composed of elongated elements arranged one-dimensionally, the
unit optical interfaces 55a may be arranged two-dimensionally.
[0072] The refractive indices of the first layer 51 and the second
layer 52 will now be described. The first layer 51 is optically
anisotropic, and is birefringent at least in a plane. Thus, the
refractive index n.sub.1x of the first layer 51 in the x-axis
direction differs from the refractive index n.sub.1y of the first
layer 51 in the y-axis direction. In addition, in the optical sheet
40 of this embodiment, the refractive index n.sub.1x of the first
layer 51 in the x-axis direction, the refractive index n.sub.2x of
the second layer 52 in the x-axis direction, the refractive index
n.sub.1y of the first layer 51 in the y-axis direction and the
refractive index n.sub.2y of the second layer 52 in the y-axis
direction satisfy the following relation:
|n.sub.1x-n.sub.2x|.noteq.|n.sub.1y-n.sub.2y|
[0073] Accordingly, the optical sheet 40 exerts different optical
effects on light of the first polarization component that vibrates
in the x-axis direction and light of the second polarization
component that vibrates in the y-axis direction. In particular,
light of the first polarization component and light of the second
polarization component, both traveling in the same direction, come
to travel in different directions after passing through the optical
interface 55 of the optical sheet 40.
[0074] Particularly in this embodiment the following relation is
satisfied:
|n.sub.1x-n.sub.2x|>|n.sub.1y-n.sub.2y|=0
[0075] In this case, the optical interface 55 of the optical sheet
40 no more functions as an effective optical interface, having a
refractive index difference, on light of the second polarization
component that vibrates in the y-axis direction. Thus, while the
optical interface 55 of the optical sheet 40 exerts an optical
effect (e.g. lens effect) on light of the first polarization
component, light of the second polarization component does not
change its traveling direction when it passes through the optical
interface 55 of the optical sheet 40. A refractive index value is
herein expressed as a value rounded off to two decimal places.
[0076] In application of the optical sheet 40 in a display device
which switchably displays a two-dimensional image and a naked-eye
visible three-dimensional image, it is not practically essential
for the refractive indices n.sub.1y, n.sub.2y to satisfy the
relation: |n.sub.1y-n.sub.2y|=0, and it is sufficient if the
following relation is satisfied:
|n.sub.1x-n.sub.2x|>|n.sub.1y-n.sub.2y| and
|n.sub.1y-n.sub.2y|.ltoreq.0.02
[0077] In this case, light of the second polarization component
will not change its traveling direction at the optical interface 55
of the optical sheet 40 to such an extent as to cause problems,
such as ghost and crosstalk.
[0078] In application of the optical sheet 40 in a display device
which switchably displays a two-dimensional image and a naked-eye
visible three-dimensional image, the level of the optical effect,
exerted on light of the second polarization component, is affected
not only by the absolute value of |n.sub.1y-n.sub.2y| but also by
other factors, including the shape of the optical interface 55 of
the optical sheet 40, as will be described in detail below. From
the above viewpoint, the optical sheet 40 may be designed so that
light of the polarization component (second polarization component)
that vibrates in the y-axis direction, traveling in a direction
perpendicular to the sheet plane of the optical sheet 40, i.e. in
the front direction, before entering the optical sheet 40, comes to
travel in a direction at an angle of not more than 2 degrees with
respect to the front direction after passing through the optical
sheet 40. This can effectively prevent an optical effect which
could cause image degradation e.g. upon display of a
two-dimensional image, due to the occurrence of a problem such as
ghost, from being exerted on light of the second polarization
component, passing though the optical sheet 40.
[0079] FIG. 4 shows exemplary refractive index ellipsoids that
indicate refractive index distributions in the first layer 51 and
the second layer 52 in varying directions. In the illustrated
embodiment the following relation is satisfied:
(n.sub.1x-n.sub.2x)>|n.sub.1y-n.sub.2y|=0
[0080] The refractive index n.sub.1x of the first layer 51 in the
x-axis direction is higher than the refractive index n.sub.1y of
the first layer in the y-axis direction. Further, in the embodiment
illustrated in FIG. 3, the second layer 52 is formed as an
optically isotropic layer. Thus, the refractive index n.sub.2x of
the second layer 52 in the x-axis direction is equal to the
refractive index n.sub.2y of the second layer 52 in the y-axis
direction. Therefore, the refractive index n.sub.1x of the first
layer 51 in the x-axis direction is higher than the refractive
index n.sub.2x of the second layer 52 in the x-axis direction.
Accordingly, the optical interface 55 shown in FIG. 1 can exert the
same lens effect as a convex lens.
[0081] In the embodiment illustrated in FIG. 1, the direction of
the slow axis, in which the refractive index is maximum, coincides
with the x-axis direction in a plane in the first layer 51, while
the direction of the fast axis, in which the refractive index is
minimum, coincides with the y-axis direction in a plane in the
first layer 51. In addition, the refractive index n.sub.1y in the
y-axis direction (direction of the fast axis) in a plane in the
first layer 51 is made equal to the refractive index n.sub.2y in
the y-axis direction in a plane in the second layer 52. Therefore,
the difference in the x-axis direction refractive index between the
first layer 51 and the second layer 52 can be set to be large while
setting the difference in the y-axis direction refractive index
between the first layer 51 and the second layer 52 to zero. In
application of the optical sheet 40 in a home display device, on
the condition that the optical interface 55 is manufactured in a
shape easy to manufacture, the birefringent index .DELTA.n
(=n.sub.1x-n.sub.1y) of the first layer 51 is preferably not less
than 0.13. On the other hand, when the optical anisotropy of the
first layer 51 is provided by stretching as described below, the
birefringent index .DELTA.n of the first layer 51 is preferably not
more than 0.22 e.g. in view of the in-plane uniformity in a
stretching process.
[0082] The refractive indices of the first layer 51 and the second
layer 52 can be measured, for example, by using "KOBRA-WR"
manufactured by Oji Scientific Instruments, "Ellipsometer M150"
manufactured by JASCO Corporation, or an Abbe refractometer (NAR-4,
manufactured by Atago Co., Ltd.).
[0083] Such an optical sheet 40 can be produced in the following
manner: First, as shown in FIG. 5, a resin film 71 is produced by
using a thermoplastic resin. Thereafter, the resin film 71 is
subjected to stretching to produce a first layer 51 composed of the
stretched resin film 71. Thereafter, a second layer 52 is formed on
the first layer 51 to obtain an optical sheet 40.
[0084] The resin film 71 can be produced by molding of a resin
material comprising a thermoplastic resin as a main component, or
consisting only of a thermoplastic resin. The molding of the resin
material may be performed by injection molding or melt extrusion.
Such a molding method can produce the resin film 71 having a
three-dimensional (corrugated) pattern that forms the optical
interface 55. As shown in FIG. 5, the resin film 71 has raised
portions 71a arranged in a direction not parallel to the
longitudinal direction of each raised portion 71a.
[0085] A mold, having a mold surface made of metal or plastic, can
be used for the molding of the resin film 71. Compared to the use
of a mold having a metal mold surface, the use of a mold having a
plastic mold surface can prevent rapid absorption of heat from a
heated thermoplastic resin into the mold surface upon application
of the thermoplastic resin onto the mold surface. This enables the
heated thermoplastic resin to fully spread over the mold surface,
making it possible to enhance the rate of shaping. Further, the
resin film 71 produced can be easily released from the mold
surface. This can prevent the formation of a defect in the resin
film 71 upon its release from the mold surface. A long film-like
mold can be used as a mold having a plastic mold surface.
[0086] Stretching of the resin film 71 is performed in order to
impart optical anisotropy to the resin film 71 and, insofar as this
object is achieved, may be performed by any of uniaxial stretching,
sequential biaxial stretching and simultaneous biaxial stretching.
When the resin film 71 comprises a polyester resin, the stretching
direction (stretching axis) coincides with the slow axis. For
example, when it is intended to make the longitudinal direction of
the unit optical interfaces 55a parallel to the slow axis of the
first layer 51, the resin film is stretched in a direction parallel
to the longitudinal direction of the raised portions of the resin
film 71 which are to form the unit optical interfaces 55a of the
optical interface 55, as shown in FIG. 5.
[0087] Stretching of the resin film 71 is carried out while heating
the resin film 71 at a temperature above the glass transition
temperature of the thermoplastic resin of the resin film 71. In the
case where the resin film 71 is produced by melt extrusion, the
high-temperature resin film 71 immediately after extrusion may be
subjected to stretching. Thus, there is no need to separately
provide a heating process for stretching of the resin film 71. As
shown in FIG. 5, the shape of the resin film 71 is changed by
stretching to form the first layer 51. Therefore, in the molding of
the resin film 71, the resin film 71 is produced in a shape that
takes into account the deformation of the resin film 71 by
stretching.
[0088] Next, the second layer 52 is formed on the first layer 51 by
applying a resin onto the first layer 51 and curing the resin on
the first layer 51. The second layer 52, thus formed on the first
layer 51, has a three-dimensional pattern, corresponding to or
complementary to the three-dimensional (corrugated) pattern of the
first layer 51, in the surface facing the first layer 51.
Alternatively, a second layer 52, which has been produced
separately, may be laminated to the first layer 51. A resin
material for the second layer 52 may be a thermoplastic,
thermosetting or ionizing radiation-curable resin which is
non-birefringent, i.e. having an isotropic refractive index
(n.sub.2x=n.sub.2y). The optical sheet 40 can be produced in the
above manner. Such a non-birefringent resin for the second layer 52
is usually solidified in the unstretched state.
[0089] The optical sheet 40 can also be produced by a production
method as illustrated in FIG. 6.
[0090] In the production method illustrated in FIG. 6, a resin film
71 having the above-described three-dimensional (corrugated)
pattern (see FIG. 5) and a second rein film 72 having a
three-dimensional pattern corresponding to, or complementary to the
three-dimensional (corrugated) pattern of the resin film 71 are
prepared first. Next, the resin film 71 and the second resin film
72 are laminated to each other e.g. with an adhesive or glue in
such a manner that the respective three-dimensional patterns engage
each other. Thereafter, the laminate of the resin film 71 and the
second resin film 72 are stretched e.g. in the longitudinal
direction of each raised portion 71a of the resin film 71 to obtain
an optical sheet 40 consisting of the first layer 51 composed of
the resin film 71 and the second layer 52 composed of the second
resin film 72.
[0091] Also in the production method illustrated in FIG. 6, the
resin film 71 and the second resin film 72 can be produced by
molding using a thermoplastic resin as in the above-described
production method illustrated in FIG. 5. Also in the production
method illustrated in FIG. 6, in-plane birefringence is imparted to
the resin film 71 by stretching of the resin film 71. Though the
second resin film 72 is also stretched together with the resin film
71, it is not necessary to intentionally impart optical anisotropy
to the second resin film 72. Therefore, in order to prevent
in-plane birefringence from being produced in the second resin film
72, the electric dipole moment of a molecule in the second resin
film 72 is preferably low. In particular, the electric dipole
moment of a molecule in the second resin film 72 is preferably at
least lower than the electric dipole moment of a molecule in the
resin film 71. The measurement of the electric dipole moment of a
film can be performed by first measuring the dielectric constant of
the film with a test fixture HP 16451B electrode of precision LCR
meter, manufactured by Yokogawa-Hewlett-Packard Ltd., and then
determining the electric dipole moment using the measured
dielectric constant.
[0092] The level of birefringence (refractive index anisotropy)
produced in a film depends on the electric dipole moment of the
constituent molecule of the film. Accordingly, by using the resin
film 71 and the second resin film 72 which satisfy the above
relation in the electric dipole moments of the respective
constituent molecules, the following relation is satisfied even
when the resin film 71 and the second resin film 72 are stretched
to the same extent to cause the same degree of molecular alignment
in the films:
birefringent index .DELTA..sub.n1 of the resin film
71>birefringent index .DELTA..sub.n2 of the second resin film 72
or, when expressed with the refractive indices of the films in the
x- and y-axis directions, n.sub.1x-n.sub.1y>n.sub.2x-n.sub.2y
(ideally.fwdarw.0).
[0093] The optical sheet 40 consisting of the optically anisotropic
first layer 51 comprising a thermoplastic resin, and the second
layer 52 which is laminated to the first layer 51 and which forms,
between it and the first layer 52, the optical interface 55 for
changing the traveling direction of light of the first polarization
component, can thus be produced.
[0094] A polycarbonate resin, a cycloolefin polymer resin, an
acrylic resin, a polyester resin, etc. can be used as the
thermoplastic resin of the first layer 51. Of these, a polyester
resin is advantageous in terms of cost and mechanical strength.
Specific examples of the polyester resin include polyethylene
naphthalate, polyethylene terephtha late, polyethylene isophtha
late, polybutylene terephtha late, poly(1, 4-cyclohexylene
dimethylene terephthalate), and polyethylene-2, 6-naphthalate. The
polyester resin, forming the first layer 50, may be a copolymer of
such a polyester resin or a resin blend of a major amount (e.g. not
less than 80 mol %) of such a polyester resin and a minor amount
(e.g. not more than 20 mol %) of other resin(s). Of the above
polyester resins, polyethylene naphthalate is preferred because it
can ensure a high birefringent index. Of the above polyester
resins, polyethylene terephthalate or polyethylene-2, 6-naphthalate
is preferred because of good balance between mechanical properties
and optical properties. From the viewpoint of stability of the
optical sheet 40, the glass transition temperature of the material
of the first layer 51 is preferably not less than 100.degree.
C.
[0095] The display device 10, which includes such an optical sheet
40, can display a two-dimensional image and a naked-eye visible
three-dimensional image in the following manner. The case of
displaying a two-dimensional image will be described first mainly
with reference to FIG. 3.
[0096] The backlight 24 illuminates an area of the liquid crystal
display panel 25 from the back. The liquid crystal display panel 25
transmits light from the backlight 24 selectively for each pixel
21. Two-dimensional image lights L31 to L36 thus formed, exiting
the image forming surface 20a of the image forming device 20, are
of the first polarization component that can pass through the upper
polarizing plate 28 of the image forming device 20. The
two-dimensional image lights L31 to L36 then enter the polarization
control device 30. When displaying a two-dimensional image, no
voltage is applied between the pair of electrodes 34, 36 of the
polarization control device 30. The liquid crystal molecules 31 are
therefore in a 90 degree-turned state as shown in FIG. 3.
Accordingly, the two-dimensional image lights L31 to L36 passing
through the polarization control device 30 change their
polarization state, and have turned into the second polarization
component when exiting the image display unit 15.
[0097] The two-dimensional image lights L31 to L36 that have exited
the image display unit 15 enter the optical sheet 40. The optical
sheet 40 has the optical interface 55 which is formed as a
corrugated surface. The optical interface 55 is formed between the
optically anisotropic first layer 51 and the second layer 52. The
refractive index n.sub.1y of the first layer 51 in the y-axis
direction, i.e. in the vibration direction of the two-dimensional
image lights L31 to L36 of the second polarization component, is
set equal to the refractive index n.sub.2y of the second layer 52
in the y-axis direction. The two-dimensional image lights L31 to
L36 therefore travel in the optical sheet 40 without changing their
travelling directions at the optical interface 55. The
two-dimensional image lights L31 to L36 then exit the display
surface 10a of the display device 10, whereby a viewer can view a
two-dimensional image.
[0098] Light from the backlight 24, illuminating the liquid crystal
display panel 25, has a light axis in the front direction (i.e. has
the peak of brightness in the front direction), while the light
travels in a direction with a certain angular range around the
front direction. Therefore, light that has passed through each
pixel 21 travels and exits the display surface 10a of the display
device 10 as divergent light in a certain angular range.
Accordingly, as shown in FIG. 3, a viewer can view the same
two-dimensional image, formed on the display surface 10a, in a
certain angular range.
[0099] The case of displaying a three-dimensional image that can be
viewed with the naked eye will now be described with reference to
FIG. 2. As with the case of displaying a two-dimensional image,
three-dimensional image lights Ll1 to Ll6, Lr1 to Lr6 exit the
image forming device 20. The three-dimensional image lights Ll1 to
Ll6, Lr1 to Lr6 then enters the polarization control device 30.
When displaying a three-dimensional image, each pixel 21 of the
image forming device 20 of the image display unit 15 is assigned to
one of those positions where the left eye or the right eye of a
viewer is supposed to be located. The image display unit 15
controls transmission and blocking of light for each pixel 21 so
that an image is formed by lights from those pixels 21 which are
assigned to the same position.
[0100] As shown in FIG. 2, when displaying a three-dimensional
image, a voltage is applied between the electrodes 34, 36 of the
polarization control device 30. Accordingly, the three-dimensional
image lights Ll1 to Ll6, Lr1 to Lr6 pass through the polarization
control device 30 while maintaining their first polarization
state.
[0101] The three-dimensional image lights Ll1 to Ll6, Lr1 to Lr6
that have exited the image display unit 15 enter the optical sheet
40. The refractive index n.sub.1x of the first layer 51 in the
x-axis direction, i.e. in the vibration direction of the
three-dimensional image lights Ll1 to Ll6, Lr1 to Lr6 of the first
polarization component, is made higher than the refractive index
n.sub.2x of the second layer 52 in the x-axis direction. The
optical interface 55 of the optical sheet 40 thus controls the
traveling direction of the three-dimensional image lights Ll1 to
Ll6, Lr1 to Lr6 from the pixels 21.
[0102] As described above, a divergent light flux from each pixel
21 enters the optical sheet 40. The unit optical interfaces 55a of
the optical interface 55 each exert a lens effect and focus a
divergent light flux from each pixel 21 on a position corresponding
to the focal point of each optical interface 55a that functions as
a lens. In particular, each unit optical interface 55a focuses a
divergent light flux (e.g. divergent light flux LF1 shown in FIG.
2), emitted from a pixel 21 located opposite to the unit optical
interface 55a, on a position to which the pixel 21 is assigned,
i.e. one of those positions where the left eye or the right eye of
a viewer is supposed to be located. The three-dimensional image
lights Ll1 to Ll6, Lr1 to Lr6 from the pixels 21 thus travel toward
their respective scheduled positions.
[0103] When a viewer views the display device 10 from a supposed
position, an image to be viewed from the position of the right eye
of the viewer can be viewed by the right eye, while an image to be
viewed from the position of the left eye of the viewer can be
viewed by the left eye. The viewer can therefore view a
three-dimensional image with the naked eye by binocular parallax.
When a viewer views the display device 10 from another supposed
position as shown in FIG. 2, an image to be viewed from that
position can be viewed three-dimensionally with the naked eye.
Thus, when the viewer changes the viewing direction, the viewer can
view different images with the naked eye according to the viewing
directions. Thus, the viewer can view an image with a higher
stereoscopic effect by motion parallax.
[0104] In a conventional display device for switchably displaying a
two-dimensional image and a naked-eye visible three-dimensional
image, a birefringent lens having an optically anisotropic layer
containing liquid crystal (liquid crystal molecules, liquid crystal
material) is used to control the traveling direction of light
depending on the polarization state of the light. The optically
anisotropic layer is typically produced by curing an ultraviolet
curable resin containing liquid crystal.
[0105] The optically anisotropic layer of the conventional
birefringent lens contains a high proportion of liquid crystal and
has a large thickness of e.g. more than 5 .mu.m in order to ensure
a sufficiently high birefringent index. Because of the high content
of liquid crystal, the conventional birefringent lens lacks
stability, especially thermal stability. This imposes restrictions
on the environment in which the birefringent lens and a display
device having the birefringent lens are installed.
[0106] According to this embodiment, on the other hand, the first
layer 51 of the optical sheet 40, having an in-plane birefringent
index, contains no liquid crystal (liquid crystal molecules, liquid
crystal material). Optical anisotropy is imparted to the first
layer 51 by stretching of the first layer 51 composed of a
thermoplastic resin. Accordingly, it is quite possible for the
first layer 51 to have a glass transition temperature of not less
than 100.degree. C. The optical sheet 40 of this embodiment and the
display device 10 incorporating the optical sheet 40 therefore
exhibit excellent thermal stability. For example, compared to the
conventional birefringent lens containing liquid crystal, the
optical sheet 40 of this embodiment can dramatically improve
dimensional stability as measured according to JIS C2151 using the
heating conditions of 150.degree. C., 30 minutes. Specifically, the
dimensional stability value of the optical sheet 40 of this
embodiment, measured according to JIS C2151 using the heating
conditions of 150.degree. C., 30 minutes, can be made as low as not
more than 2%. The optical sheet 40 of this embodiment can therefore
be used, without significant restriction on it, in a common
environment where a home television receiver, for example, is used,
and the optical sheet 40 can exert the expected optical effect.
[0107] Various changes and modifications may be made to the
above-described embodiment. Some variations will now be described.
In the following description, the same reference numerals are used
for the same members or elements as used in the above-described
embodiment, and a duplicate description thereof will be
omitted.
[0108] The optical sheet 40 is merely an example and can be
arbitrarily changed: The film layer 43 is not essential and may be
omitted from the optical sheet 40. An additional film layer, which
is expected to perform a certain function, may be provided at a
position nearer to the polarization control device 30 than the
first layer 51 and the second layer 52. As described above, the
construction of the optical interface 55 and the unit optical
interfaces 55a can be arbitrarily changed depending on a desired
optical effect. Further, the optically anisotropic first layer 51
may be disposed nearer to the viewer than the second layer 52.
[0109] The above-described relation between the refractive index
n.sub.1x of the first layer 51 in the x-axis direction, the
refractive index n.sub.1y of the first layer 51 in the y-axis
direction, the refractive index n.sub.2x of the second layer 52 in
the x-axis direction and the refractive index n.sub.2y of the
second layer 52 in the y-axis direction is merely exemplary, and is
not intended to limit the scope of the present invention.
[0110] In the above-described embodiment the refractive index
difference between the first layer 51 and the second layer 52 is
made zero in either one of the x-axis direction and the y-axis
direction. However, it is possible to make the refractive index
difference between the first layer 51 and the second layer 52 not
zero in both of the x-axis direction and the y-axis direction. Also
in this case, the same effect as described above can be obtained by
appropriately designing the construction of the optical interface
55 and the unit optical interfaces 55a.
[0111] Though in the above-described embodiment the main axes (the
slow axis and the fast axis) in a plane of the first layer 51
coincide with the directions of vibration of light that forms a
three-dimensional image and light that forms a two-dimensional
image, it is possible not to make the main axes coincide with the
vibration directions. Also in this case, the same effect as
described above can be obtained by appropriately adjusting the
refractive indices n.sub.1x, n.sub.1y, n.sub.2x and n.sub.2y.
[0112] The modifications described above can of course be made in
an appropriate combination to the above-described embodiment.
* * * * *